As a Green I get very annoyed by political parties' green claims when it's so often just wordy greenwash, not backed by action. For example, Liberal Democrat environment policy is one of the great enigmas of modern British politics. The Lib Dems so often proclaim good policy, and so commonly don't stick to it. It wouldn't be unfair to say that what's most consistent about them is their inconsistency. And they have remained true to this principle historically.

Back in the 1990s Paddy Ashdown's Lib Dems wanted a moratorium on roadbuilding. But they wholeheartedly supported the Newbury bypass, the Batheaston bypass, and so on, right up to the M74 extension in Scotland. This year Norman Baker has been saying a Lib Dem government would stop spending on roadbuilding; but his colleagues in Lancashire are still supporting the Lancaster northern bypass. They have spoken in favour of congestion charging nationally, but against it in Edinburgh, Manchester and York.

They have a tendency to say one thing at the national level and do something else at the local – though not consistently. In the 2002 local elections the Lib Dems lost control of Sheffield council by arguing for a new incinerator, and gained control of Hull by campaigning against an incinerator. They currently support incinerator projects in Exeter, Plymouth and Barnstaple, and also in Essex, despite having proclaimed support for a zero waste strategy – which means no incineration.

They want a zero carbon economy by 2050 – in principle. But they have opposed windfarm proposals in Cornwall, Cumbria, Devon and Worcestershire. In Lewisham they recently voted against a Green party budget package that would have insulated 25,000 homes for free. And when it comes to aviation, the fastest growing source of greenhouse gas emissions, it varies.

The Lib Dems have opposed the expansion of Heathrow, but have been happy to expand Birmingham, Carlisle, Exeter, Liverpool and Norwich airports. They were wildly enthusiastic about Manchester airport's second runway – except in Stockport, which lies under the flightpath. And then, having supported a £172m second runway, doubling Manchester airport's business in the space of a decade, the local Lib Dems have recently been campaigning against a new airport warehouse, in order to save a local cottage. The Save Rose Cottage campaign features in their local publicity as an example of their environmental credentials.

I have to say they really don't get it. In January this year Norman Baker was complaining, quite reasonably, that British rail passengers pay the highest fares in Europe. Then he said his party would improve matters by freezing UK rail fares. (At the highest level in Europe.)

It seems the Lib Dems know much more about pursuing the Green vote than about pursuing Green policies. In their "green tax switch" announced last September, they promised to "cut income tax and switch to green taxes on pollution instead". No serious Greens would contemplate this. We rely on income tax to fund schools, hospitals and public services. If we replace this funding with eco-tax revenue, then either we have to rely on keeping the pollution going, so as to keep the revenue coming in, or else we have less money for schools, hospitals, public services and so on.

No, they don't get it, and that's a huge annoyance to those of us who do.